Burglar alarms often require connecting links such as AC power supply and telephone lines which make them vulnerable and easily defeated by experienced burglars by cutting said lines. Even if the receiver is designed to detect the cutting of the telephone line, the alarm system can be defeated by first splicing an external shunt across the telephone line prior to the cutting thereof.
To avoid the above problems and to eliminate the substantial cost of maintaining a leased telephone line (unless an automatic dialer is included) which also limits the distance between transmitter and receiver, many prior art alarm systems use a radio transmitter. Exemplary patents using radio transmitters are: Fowler, U.S. Pat. Nos. 4,012,728; Solomon, 3,889,250; Isaacs, 3,795,896; and McQuown Jr., 3,760,359. However, radio transmitters are susceptible to interference and to being defeated by jamming or otherwise making the designated transmitter frequency unusable so that the alarm signal will not be received or be usable by a distant receiver. If multiple transmitters are used to broadcast the alarm over a plurality of frequencies such an arrangement presents a considerable extra expense. The present invention overcomes this difficulty by providing a single transmitter having a plurality of transmitting frequencies. The frequencies are switched and transmitted in seriatim thereby using only one transmitter to broadcast on a plurality of frequencies and thereby overcoming the potential jamming problem and providing a much more economical way for broadcasting on a plurality of frequencies than to use a plurality of transmitters.
Once an alarm condition is detected, it is common practice to transmit a verbal alarm signal which has been previously recorded. Such examples in the prior art are exemplified in Smith, U.S. Pat. Nos. 3,596,024 which uses a tape player using an endless tape which is continuously repeated, and Andrews, 3,207,849 and Stradley, 2,847,507 which use a phonograph.
McCorkindale, U.S. Pat. No. 3,257,653, shows transmitting a communication which requires decoding equipment or a computer for a read out.
McCorkindale does not show a tape player which can play an audible comprehensible message as well as a greatly speeded up message compressed in time for communications with a computer but not comprehensible to a human. The increased rate has the advantage of using the computer capability more effectively and not wasting computer time and additionally, has the very important advantage that if the transmission is being monitored by the intruders, it will not make sense to them and they will not be forewarned of the danger that police authorities will be arriving shortly. Further, if the burglars intend to jam the transmitter only when the message is transmitted, the message will be transmitted and finished before the intruders have time to react.
Accordingly, it is an object of the present invention to provide an apparatus and method overcoming the above problems.
A further object is to provide an alarm system that virtually cannot be jammed or disabled. Still another object is to provide an alarm system and method therefor including a radio transmitter wherein the carrier frequency of said transmitter is repetitiously and cyclically switched between a plurality of frequencies in seriatim so that if at least one of the frequencies is rendered unusable by the burglars or the like, the other of said frequencies will still be usable and the indication of an alarm condition will be transmitted and received by a distant receiver. Yet another object is to provide an alarm system and method therefor including a magnetic tape player having a pre-recorded message wherein the tape player is provided with a speed adjustment means so that the playback speed of the pre-recorded message can be greatly increased so that it is not comprehensible to a human but can be understood by a computer. Further objects and advantages of the present invention will become apparent as the following description proceeds and the features of novelty characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.